This invention relates to electrostatic developer compositions and to an improved imaging method using the compositions. More particularly, it relates to liquid negative developer compositions of improved stability and an extended service life.
Conventional liquid developers for use in electrostatic copying machines consist of an organic nonpolar liquid carrier having a low dielectric constant and high resistivity containing a toner comprising a solid particulate resinous fixative and a pigment or pigment system. A charge control agent and one or more substances for enhancing the shelf-life of the composition and for maintaining the various solid components as a homogeneously dispersed phase are also included. When a substrate containing a latent electrostatic image is brought into contact with the developer composition, charged components of the developer are attracted preferentially to the oppositely charged latent image and subsequently fixed, typically by the application of heat to evaporate the carrier, to produce a permanent visible image.
In an ideal developing composition, the fixative and pigment should be intimately associated, of uniform small particle size, and should be uniformly charged. This would result in uniform depletion of the toner as images are developed sequentially and in uniform density of the successively produced copies. In practice, this ideal property of developing compositions has been difficult to achieve. The static charge imparted to the solid particles in such a composition by the charge control agent is typically a function of the chemical properties of the agent and the toner particles and of the surface area of the particles. Thus, relatively small differences in particle size result in particles of varying charge, and in use, the larger particles in the composition are preferentially depleted. As a result, the image density of successively produced copies decreases since a given charged area of the latent image on the substrate attract a substantially constant charge, but that quantity of charge is associated with a smaller mass of toner. Also, since the majority of liquid developing compositions contain vehicle-soluble charge control agents, and since the charge control agent is depleted to a lesser extent than the fixative and pigment, as successive copies are produced the net charge on particles remaining in the developer varies in a complicated way resulting in variations in the image density of the copies.
The prior art teaches various approaches to solving this problem, but none have been wholly successful. Currently available copying machines are equipped with means for monitoring the particle density of liquid developers. When the particle density falls below a selected level, developer concentrate and/or vehicle is added to the working developer suspension to adjust the particle density to more optimal levels. However, the image density of successively developed copies nevertheless decreases since the proportion of optimally charged larger sized particles in the working developer becomes smaller. At a point when the image density of the copies falls below an acceptable level, additional relatively large quantities of vehicle are added to the working developer mix, typically by a key operator or a service representative. The result is a marked decrease in the particle density as read by the detector. This low particle density reading triggers the introduction of a relatively large quantity of toner concentrate. Thus, the proportion of ideally charged particles in the developer composition and the image density of subsequently produced copies are sharply increased, but still does not attain the level achieved by fresh developer. As additional copies are made, the developer again becomes gradually depleted, and the cycle of piecemeal replenishment followed by a sharp increase on addition of more vehicle is repeated. After several such cycles, and typically in the 10,000+ copy range, the developer no longer produces copies of acceptable image density and can no longer be upgraded sufficiently. This necessitates removal of the depleted developer and replacement with a completely fresh batch.
To graphically illustrate this phenomenon, image density may be plotted as a function of the number of copies produced. Such a plot, as depicted in the drawing, shows a gradual decrease in image density as the developer becomes preferentially depleted, despite the piecemeal replenishment of toner, followed by a sharp increase in image density following the addition of a large quantity of developer, and plural repetitions of the cycle at lower image densities until the image density is unacceptable.